In the design of electronic devices, impedance matching between the input impedance of an electrical load and the fixed output impedance of the signal source to which it is ultimately connected is of crucial importance to ensure the correct functionality of the device. Efficiency of power transmission between a source and a load may be reduced by power reflection from the load back to the source. In other words, if the impedance between a source and a load is not correctly matched, only a portion of the power is ultimately transferred to the load while the remaining power is reflected back to the source. This is a problem in the design of high-frequency electronics where it is desired that the radio frequency power is efficiently transmitted into the load so as to minimize extra power loss and signal distortion, which may negatively impact the overall performance of the system.
One method to ensure that the load receives adequate power is to increase the power transferred from the source to the load, so that the load, by means of brute force, receives adequate power. However, this method provides greater reflected power and exacerbates damage to the power source.
An alternative solution is to provide between the source and the load an impedance matching network, which may be designed such that the impedance of the source is matched to the impedance of the load. In this way, an efficient transmission of power between the source and the load can be effected while minimizing power reflection. In many applications, the impedance of the source or the load may vary over time, thereby requiring the impedance matching network to be tunable so as to ensure the efficient transmission of power between the source and the load. This may be achieved by providing a tunable impedance network containing one or more tunable impedance sub-networks, such as the one shown in FIG. 1. The conventional tunable impedance network 10 of FIG. 1 consists of a metal wire 11 and a slidable impedance element 13 that can be moved continuously up and down the wire 11. By sliding the impedance element ZTUNE 13 up and down the length of the wire 11, as indicated by the arrows, the impedance parameters seen at the two ports of the tunable impedance network 10 can be adjusted.
Such a tunable impedance network is not suitable for integrated circuit solutions due the large area overhead required for its implementation and the need for manual control of the slidable impedance element 13. Similar functionality can be approximated with a discrete set of switchable impedance elements in a switched capacitor array. By switching one of the elements, a capacitor can essentially be moved up and down along the wire. While this solution of moving an impedance up and down a wire is technically feasible, it is very inefficient. It requires a large layout and number of unit elements, all of whom generate a certain amount of harmonic distortion (even when switched off) due to various unavoidable non-linear capacitances present in the physical layout. Furthermore, the tuning range may be quite limited, since the inductance of the wire may be less than a nano-Henry, depending on the frequency, even for a relatively long length of integrated wire.
Using such a switchable capacitor array as a conventional digital-to-analog converter (DAC), e.g. a thermometer-coded DAC, may provide a much larger tuning range. The maximum amount of capacitance that can then be connected to the wire is then n*CUNIT. However, the capacitance connected to the wire is influenced by routing inductance of the wire, which may, together with process variations, affect the precision of the switched capacitance, which is of crucial importance when the tunable impedance network is used for impedance balancing in electrical-balance duplexers. In this frequency-flexible type of duplexer, the basic concept is to create a so-called balance network impedance, the complex impedance value of which enables destructive signal interference at the receive output of a hybrid transformer. Specific to this application is the need of an extremely high accuracy in the impedance, as well as a high number of degrees of freedom or independently tuned impedances that enable highly accurate signal cancellation at an output of the duplexer.
Thus, there is a need for additional fine tuning of such a variable impedance bank.